The former Hull City man, who joined Boro at the beginning of the campaign, has been dogged all season by an ankle injury.
He has taken his place on the bench a few times since returning to full fitness, but Annan has now opted to make the switch down the A64.
Boss Steve Kittrick has underlined that this is likely to be a short-term switch, as Boro will once again have a look at Annan in pre-season.
Kittrick said: "We were looking at Will going out on loan to play football, but with Tadcaster being at the same level he had to join them on a permanent basis.
"I left it up to Will and he has decided to go and get some first-team football.
"He has been out for a long time, so he needs to get some games under his belt.
"We will keep tabs on him and then have a look again in pre-season."